The sun was shining brightly yesterday on Union Square where hundreds of Chinese Americans gathered to commemorate this week's 50th anniversary of V-J Day -- victory over Japan in World War II.

But for Paul Hsu, the celebration was time to reflect on a dark episode, the day in December 1937 when two Japanese soldiers stormed into his family's home in the Chinese village of Wuxi and tried to rape his 20-year- old sister, Kui Ling.

"She broke away and ran out of the house," the 76-year-old Hsu recalled, his eyes welling with tears. "They chased her to the Yangtze River, which ran behind our house. When she tried to escape along the bank, she fell in the river. We never saw her again."

For the Chinese people who lived through Japan's occupation, and for their children and grandchildren who have heard the tales of Japanese atrocities, World War II remains an open wound.

On Aug. 15, 1945, Emperor Hirohito broke the news of Japan's surrender to his people, many of whom were stunned because the government's tight control of information had prevented them from knowing the full extent of their country's losses.

Hsu and the others who gathered for the rally in Union Square and subsequent parade through Chinatown are waiting to hear something else: They want Japan to apologize for the 24 million to 30 million people that Chinese historians estimate were killed during the 14-year Sino-Japanese war that began in 1931 and lasted until the end of World War II.

"They need to admit what they did, like the Germans have," Hsu said. "They need to stop hiding the truth from their people."

Last month, Japan's Prime Minister Tomiichi Murayama issued an apology for his country's wartime actions in forcing women into prostitution. A month earlier, the lower house of Japan's parliament passed a resolution expressing "deep self-reflection" over Japan's wartime past.

The resolution said that many countries engaged in "colonial rule and aggressive-like acts," and that in this historical context, Japan may have committed such acts as well.

Conservatives in parliament argue that an apology to Japan's victims would dishonor its war dead. Liberals note the United States has not apologized for the atomic bombings of Hiroshima and Nagasaki.

"They're trying to rewrite history," said Gilbert Chang, past chairman of the Alliance for Preserving the Truth of the Sino-Japanese War, a Northern California group that helped organize the more than 40 Chinese American groups that participated in yesterday's commemoration. "We want a full apology."

The alliance has sponsored a national tour of a photo exhibit, "The Forgotten Holocaust," which covers the systematic biological research conducted by Japanese doctors on Chinese people and the servitude of as many as 200,000 young women as sex slaves.

The group also has called for a one-month boycott of products made in Japan and has published a pictorial book, "Rape of Nanking."

Three days after after Hsu's sister was chased into the Yangtze River, the same Japanese soldiers marched into the then-Chinese capital of Nanking, now Nanjing. In the next four months, they killed an estimated 300,000 civilians. This chapter in history is little- known outside the Asian communities.

"When we study history in our schools," San Francisco school commissioner Leland Yee told yesterday's gathering, "they don't talk about Nanking. We need to know that our children will know so that we cannot forget."